The brightness of light emitting diodes (LEDs) is directly dependent on the load current flowing through the diode. To vary the brightness of a LED it is known to use a controllable current source that is set to a current representing a desired brightness. In digitally controlled applications a digital-to-analog converter (DAC) may be used to set the current of the controllable current source.
Since the human eye cannot resolve high frequency brightness fluctuations of approximately 100 hertz or higher, it is known to supply the LED with a pulse width modulated (PWM) current of sufficient frequency. In this case the human eye low-pass filters the resulting pulse width modulated brightness of the LED, i.e., the eye can only sense a mean brightness that depends on the mean LED current which is proportional to the duty cycle of the pulse width modulation. Consequently only the mean current through a LED is relevant to the brightness perceived by the human eye.
It is known to combine light of different colors (e.g., red, green and blue) each having variable brightness to generate nearly any color sensation in the visible spectrum of light. In modern illumination systems or displays a combination of at least three LEDs of different colors are used to provide a multi-color illumination. The LED-triples may be arranged in a matrix-like structure thus forming a display where each “pixel” of the display is represented by a LED-triple typically comprising a red, a green and a blue LED. To vary the color of a pixel, the brightness of the different LEDs has to be individually adjustable. Each of the three LEDs may therefore be driven by a pulse-width modulated current signal of a sufficient high frequency, for example 400 hertz.
However, the resolution requirements are quite high for modern illumination systems or displays. That is, the brightness of a single LED should be adjustable to at least 4,096 different brightness values which corresponds to a brightness resolution of 12 Bit. When using pulse width modulation for controlling the brightness, a time resolution of approximately 600 nanoseconds has to be provided in order to be able to resolve a PWM period of, for example, 2.5 milliseconds (corresponds to 400 hertz) with 12 bits. This entails the need for very fast switching currents with all the known problems coming therewith. Particularly, the electromagnetic compatibility (EMC) is low when switching currents with rise and fall times in the sub-microsecond range.
Further, the brightness of each individual LED is subject to a thermal drift which leads to a respective color drift in a multi-color application. Varying the current in response to a temperature variation to compensate for the effects of the temperature drift is not satisfying since the wavelength of the color of a single LED may vary in response to a changing LED current. Thus, a very complex brightness control would be necessary in multi-color LED systems since the color has to be corrected when changing the brightness of a three LED pixel.
Generally, there is a need for an alternative concept for driving LEDs and multi-color LED-arrangements, particularly for LED drivers that provide an improved color stability over a wide temperature rage.